The present invention relates to a control system for an automatic clutch interposed between an engine and a transmission of a motor vehicle, and particularly to a system for controlling the clutch at engine braking.
Recently, the automatic clutch is employed in the motor vehicle for smoothly transmitting power of the engine to an automatic transmission. The automatic transmission is controlled by a control system for controlling clutch torque.
There is provided an automatic transmission having a high reduction ratio range (Ds-range) for effecting the engine braking at any speed of the transmission. When the Ds-range is selected during down-hill driving, engine braking is effected by the high reduction ratio of the transmission. Thus, safe driving is insured. However, it is necessary that the vehicle is on a road having as a high a friction coefficient .mu. as a dry road for the engine braking.
If the Ds-range is selected during driving on such a slippery surface of the road as a snowy road having a low friction coefficient .mu., the large engine braking force becomes larger than the gripping force of the tires for the road surfaces so that the corresponding vehicle wheel is locked. Since the wheel skids, the wheel speed is rapidly reduced, resulting in a loss of steering the vehicle.
In the transmission having the automatic clutch, the clutch is temporarily disengaged to reduce the engine braking force. Thereafter, when the vehicle enters a dry road zone, the gripping force of the tires is restored. However, since the clutch is disengaged, the engine braking is not effected. Therefore, it is necessary to quickly control the clutch in dependency on the gripping force.
Japanese Patent Application Laid-Open 61-129330 discloses an automatic transmission system in which if the difference between the rotating speeds of the driving wheels exceeds a predetermined value, the clutch torque of a clutch is reduced.
In the system, the clutch is controlled when the wheels slip at starting or at acceleration. Therefore, the system is not available for control at the engine braking.